In the production of seamless metal tubing, cylindrical billets, heated to an appropriate temperature, are simultaneously rotated and advanced over a mandrel-mounted piercing point. The billet is thus formed into a hollow seamless tube, which is thereafter processed by other mills, in which it is elongated, reduced to desired diameter and wall thickness, and eventually cut to desired length. As will be appreciated, the accuracy, or lack of it, with which the initial piercing operation is carried out, is reflected throughout all the subsequent processing operations and in the finished tubing. Thus, the more precisely the original piercing point is aligned with the geometric axis of the billet, the more uniform will be the walls of the tubing and the higher the quality of the finished product.
In order to properly start the advancing of the billet over the piercing point, it has been conventional for many years to form a centering recess in the end of the billet, which serves to align and stabilize the piercing point as the piercing operation is commenced. In recent years, hydraulic billet centering equipment has been widely used for this purpose, in which a hot metal billet is held by a clamp, and a centering punch is hydraulically advanced against the billet to provide the desired centering indentation.
Metal billets for seamless tube production are conventionally produced by a variety of techniques, among which are continuous casting, extrusion, hot rolling, and turning. In any of these operations, there are inevitable variations from the nominal outside diameter of the billet. The overall variation in diametral dimension will, of course, be a function of individual mill practices, but even in the most efficient and quality conscious mills, variations are bound to occur in the initial production of the billets. Additionally, prior to center punching, the billets are heated to piercing temperature, typically around 1150.degree. C. This of course results in an increase in the diameter of the billet, which may vary somewhat from billet to billet, depending on the particular alloy composition.
In accordance with the present invention, a novel and improved hydraulic billet centering machine is provided, which includes means for automatically adjusting the position of the center punching tool in relation to the actual diameter of the billet, such that the center punch is at all times accurately related to the actual geometric axis or center line of the billet. More particularly, an improved center punching apparatus is provided, which includes means for axially advancing a center punching tool, wherein the tool is mounted for limited transverse adjustment along a plane including the geometric axis of a billet clamped in the equipment. The transverse adjustment of the center punching tool is independent of its position of advancement or retraction in the direction of the billet axis and is automatically related to the actual diameter of the billet being centered.
In accordance with another specific aspect of the invention, a hydraulic billet centering machine is provided, as described in the preceding paragraph, in which the transverse adjusting mechanism for the center punching tool is mechanically connected to a billet clamping mechanism. The clamping mechanism, which is arranged to grip the billet from opposite sides and includes one fixed and one movable clamping die, is inherently responsive to the actual size of the billet, having a greater closing movement onto a smaller diameter billet than a larger diameter billet. This inherent sensing action of the clamping means is directly utilized to provide a fine adjustment of the center punch position, so that an exceptionally high degree of centering accuracy is achieved in normal mill production operations.
For a more complete understanding of the above and other features and advantages of the invention, reference should be made to the following detailed description and to the accompanying drawings.